More than half the money donated to Boris Johnson's mayoral campiagn came from the financial sector. Photograph: Nils Jorgensen/Rex Features

Boris Johnson, who is leading the fight against a European crackdown on City financiers, faced accusations of being "bought off" today, when it emerged that more than half the money donated to his mayoral campaign came from the financial sector including hedge funds and private equity.

Johnson has criticised the EU's so-called "hedge fund directive," draft rules published in the spring which would limit debt levels for alternative investment managers, such as hedge funds, and force them to be more transparent.

But research by the Party of European Socialists (PES), which is supporting tougher regulation, shows that Johnson received £324,000 from City grandees between September 2007, when he was endorsed as the Conservative party's candidate, and March 2008 when the mayoral race kicked off. That was more than half of the total £584,701 in donations he received – and 77% of the total campaign spending he was allowed by law.

Poul Nyrup Rasmussen, the former Danish prime minister and PES president, said: "Hedge funds are buying off the Tories and Boris Johnson, which shows that they want to remain above the law. This will reinforce democratic efforts to introduce better regulation in the wake of the financial crisis."

Conservative party coffers have also been boosted by the alternative investment industry. The PES calculates that in the first six months of 2009, donations worth £3.26m – 46% of all individual contributions to the party – came from managers at City firms, the majority of them hedge funds, with a further £382,500 coming from the companies themselves. The largest single donation was £1.08m from Stanley Fink, who made a fortune building up the hedge fund Man Group before leaving last year to become co-treasurer of the Tory party.

John Monks, general secretary of the European Trade Union Confederation, said: "There's a furious lobbying effort going on, particularly from London. A lot of people have a vested interest in this, including Boris Johnson."

A spokeswoman for Johnson said his views had cross-party support, and, "the mayor believes the stakes are still very high." London is the European capital of hedge funds, with up to 80% based in the City, and Johnson has warned that they will move offshore if the directive is passed.

Lord Myners, the City Minister, has also been critical of the directive, which is widely expected to be significantly re-drafted by the European parliament in the coming months, before being considered by EU finance ministers, with the aim of introducing legislation by next spring. A Treasury spokesman said the government is convinced that all "systemically important" financial institutions, including hedge funds, must be regulated at European level, but Brussels should not take a "one size fits all" approach.

Glenis Willmott, MEP for the East Midlands and Labour leader in the European parliament, said: "Lobbying is fine, as long as it's transparent what's happening. It's clear we need to have regulation, but it's a long way to go before we have a directive we can all live with."